1. Field of the Invention
The present invention relates to a rear wheel steering device, and more particularly to a rear wheel steering device including an electric motor for steering rear wheels in a four-wheel-steerable vehicle.
2. Description of the Relevant Art
One known motor-operated rear wheel steering device in a four-wheel-steerable vehicle is disclosed in Japanese Laid-Open Utility Model Publication No. 61-31972. The disclosed motor-operated rear wheel steering device includes a rod member extending transversely of the vehicle body and having opposite ends coupled to tie rods for turning respective rear wheels, and a ball screw mechanism combined with the rod member. An electric motor has an output shaft extending parallel to the rod member, and the ball screw mechanism has a nut member, with a speed reducer mechanism interposed between the nut member and the output shaft.
Because the rod member and the output shaft lie parallel to each other, this conventional motor-operated rear wheel steering device suffers the following problems: The gear box has large outer dimensions, imposing limitations on the spaces for storing the fuel tank below the rear seat and the spare tire below the trunk. Since the speed reducer mechanism projects from one side of the gear box, making the gear box complex in shape, the gear box must be formed by a costly casting process. It is necessary to keep parallelism between the rod member and the motor output shaft and their dimensional tolerances at high accuracy, so that these parts should be machined by a costly precision machining process.
Japanese Laid-Open Patent Publication No. 59-50864 discloses a rack-and-pinion front wheel steering device employing an electric motor for producing assistive steering power. The disclosed rack-and-pinion front wheel steering device includes a rack shaft on which the stator and rotor of the motor are mounted, and a ball screw mechanism disposed between the rotor and the rack shaft for converting rotation of the rotor to linear motion of the rack shaft. The rack shaft has rack teeth on one end portion thereof which are held in mesh with the pinion gear on a steering gear input shaft. The ball screw mechanism is disposed on the other end portion of the rack shaft. The ball screw mechanism includes a ball nut around which the rotor of the motor is disposed. Therefore, the rotor of the motor is spaced radially outwardly from the outer periphery of the rack shaft by a distance substantially corresponding to the wall thickness of the ball nut.
In the case where the above conventional steering device is employed as the rear wheel steering device in a four-wheel-steerable vehicle, the following considerations must be taken into account: Since the rotor of the motor is spaced from the outer periphery of the rack shaft, the moment of inertia of the rotor is comparatively large and hence the rotor tends to start rotating with a delay in response to energization of the motor. This does not cause any problem in the front wheel steering device in which the wheels are turned manually through the rack-and-pinion mechanism. However, since the rear wheel steering device is operated by the motor only, the above problem is quite important in the rear wheel steering device since it creates a delayed response in the rear wheel steering action. Another drawback is that since the ball screw mechanism necessarily has axial play or backlash between the ball nut and screw due to structural limitations, it is possible for the rear wheels to be turned under external forces when the rear wheels are in a steered or neutral condition. This play can be cancelled out by the rack-and-pinion mechanism in the front wheel steering device. However, it is of great consequence in the rear wheel steering device to eliminate such play since the rear wheel steering device has no rack-and-pinion mechanism. Japanese Laid-Open Utility Model Publication No. 61-31972, as referred to above, discloses a lock device for preventing the rotor of the motor from rotating while the rear wheels are in the neutral condition. However, the rear wheel steering device is not free from external forces applied while the rear wheels are being turned.
The present invention has been made in view of the foregoing difficulties.